In optical fiber communication systems, in order to increase the reach and the rate of optical transmission, the optical signal-to-noise ratio has to be increased. Thus, a decrease in transmission losses in optical fibers is demanded. Nowadays, since an optical fiber production method is highly sophisticated transmission losses caused by impurities contained in optical fibers are closed to the lower limits. A remaining main cause of transmission losses is scattering losses in association with fluctuations in the structure or composition of glass forming optical fibers. This is inevitable, because optical fibers are formed of glass.
As a method of decreasing fluctuations in the structure of glass, a method is known to cool molten glass slowly. As a method of slowly cooling molten glass, an attempt is made to slowly cool an optical fiber drawn from a drawing furnace immediately. Specifically, it is investigated to decrease the cooling rate of the optical fiber that an optical fiber drawn from a drawing furnace is heated in an annealing furnace, or an optical fiber drawn from a drawing furnace is surrounded by a heat insulator immediately.
Patent Literature 1 below discloses a method of setting the temperature of a heating furnace (an annealing furnace) is ±100° C. or less of the target temperature found by a recurrence formula in 70% or more of a region from a position at which the outer diameter of a silica based optical fiber having a core and a cladding becomes smaller than 500% of the final outer diameter to a position at which the temperature of the optical fiber is 1,400° C. Since the temperature history of the optical fiber is controlled in this manner, the fictive temperature of glass forming the optical fiber is decreased to reduce transmission losses.    [PATENT LITERATURE 1] JP2014-62021 A
However, the technique disclosed in Patent Literature 1 above is required to repeat complex calculations in order to adjust the temperature of the optical fiber to an ideal temperature change found by the recurrence formula. The technique disclosed in Patent Literature 1 permits the temperature of the optical fiber to have a temperature shift of as large as ±50° C. to 100° C. with respect to the target temperature found by the recurrence formula. When the temperature shift of the optical fiber is permitted in such a large deviation, it is difficult to say that the temperature history is sufficiently optimized. For example, supposing that the temperature of the optical fiber slowly cooled is changed in a range of ±100° C. and the fictive temperature of glass forming the optical fiber is also changed in a similar range, transmission losses of the obtained optical fiber caused by light scattering fluctuate as large as about ±0.007 dB/km. In such the disclosed production methods in which the temperature history of the optical fiber is not sufficiently optimized, the annealing furnace is elongated more than necessary, resulting in excessive capital investment, or the drawing rate is decreased more than necessary, resulting in degraded productivity.
The present inventors found that the temperature difference between the fictive temperature of glass forming the optical fiber and the temperature of the optical fiber is controlled in a predetermined range in the slow cooling process, resulting in the promotion of the relaxation of the structure of glass forming the optical fiber and easily reducing transmission losses in the optical fiber.